1. Field of the Invention
The apparatus and method of the present invention relate to a device used to furnish high pressure power fluid for actuation of hydraulically actuated valves or other hydraulically operated devices located beneath the surface of a body of water.
2. Description of the Prior Art
As well drilling operations progress into deeper waters in the search for new oil and gas reserves, new subsea equipment must be developed. Valves are the main flow control devices for this equipment. Since handwheel operated valves cannot easily be used below the surface of bodies of water, hydraulically actuated valves (and other pressure and flow control devices) are typically used to control the flow of oil and gas from underground reserves.
A hydraulically-actuated valve typically has a valve actuator, connected to a moveable valve gate carried within the valve body. Pressurized hydraulic fluid supplied to the valve actuator acts on one side of a moveable piston or diaphragm connected to the moveable valve gate. The force generated by this pressurized fluid causes the gate to move into an open or closed position.
This pressurized hydraulic fluid is typically generated by high pressure, low volume, positive displacement pumps. Since a large volume of hydraulic fluid is required to activate most subsea valves, and other pressure control devices, pressurized hydraulic power fluid reservoirs, or "accumulators", are used in conjunction with the low volume positive-displacement pumps. The pump will be operated for a sufficient length of time in order to supply the accumulator with the amount of pressurized fluid required to operate the valve quickly, when needed.
These accumulators usually take the form of a hollow metal spherical or cylindrical canister partially filled with a pressurized inert gas, such as nitrogen, and partially filled with a pressurized hydraulic fluid. In operation, these accumulators are initially precharged with pressurized nitrogen prior to being submerged. The precharged pressure usually is equal to the anticipated pressure of the water that will be encountered at the depth of submersion of the accumulator. This precharging is necessary to provide a compressible medium that will accept a quantity of power fluid upon charging, and then expell it upon demand.
Upon reaching the operating water depth, the canister is charged with an additional increment of pressure equal to the system differential operating pressure that is required to open or close the valve. This pressure will be roughly equivalent to the pressure required to operate the valve at atmospheric pressure conditions, if the valve were still above the surface of the water.
Unfortunately, should this accumulator not be relieved of operating pressure before its recovery to the surface, extremely high differential pressures between the interior and exterior of the accumulator will result. The accumulator must either be built to with-stand these differential pressures, or the risk of a burst at the surface must be tolerated, with risk of loss to the crew of the surface vessel.
In any event, existing accumulators for deepwater valve activation are typically very bulky, heavy, and provide small working volumes of pressurized fluid. For example, if an accumulator having a volume of 25 gallons were used to activate a valve located in 8000' of water, and the actuator of the valve or other device required at atmospheric pressure a 3000 p.s.i. system operating pressure, only 3 gallons of pressurized hydraulic fluid would be delivered from the 25 gallon accumulator.
A subsea power fluid accumulator therefore is needed that does not present a potential safety hazard to a surface vessel and its crew. This apparatus should also efficiently use pressurized hydraulic fluid accumulated within its boundaries to quickly operate any subsea hydraulically-activated valve assembly.